Apparatus for coating a structure with a uniform foam layer

ABSTRACT

An apparatus for forming a layer of low density foam of uniform thickness on a surface is disclosed. A plurality of upstanding studs spaced across the surface to be coated support a taut screen at a selected height above the surface. An applicator applies liquid, self-rising foam material in a uniform manner to said surface through said screen. As the foam rises, it penetrates through the screen, leaving irregular projections above the screen. After the foam cures to at least a self-sustaining state, the screen is stripped away, leaving a uniform foam layer. Since the screen offers very little resistance to the rising foam, the resulting foam layer has a very uniform low density, highly desirable for thermal insulation application.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. Pat.Application Ser. No. 502,586, filed Sept. 3, 1974, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates in general to the application of foam coatingssuitable for thermal insulation, and more specifically to the formationof low density foam coatings of uniform density and thickness.

Highly efficient thermal insulation materials are becoming increasinglynecessary for applications such as cryogenic fuel tanks in high energyspace launch vehicles, liquefied natural gas storage tanks, ships fortransporting liquefied natural gas, and other similar applications. Theinsulation must be highly efficient, light in weight, and reasonablyinexpensive to apply and maintain over very large structures.

Many insulation systems are in use today. While many, such as asbestospacking, glass fiber batts, etc., are useful when the temperaturedifference within and without an insulated structure is not great, mostof these are unacceptable for use with cryogenic tanks where thistemperature difference is in hundreds of degrees. Insufficientinsulation results in, for example, an undesirable waste of cryogenicliquids due to inward heat transfer and the resulting boil-off andventing of part of the liquid.

A number of "super insulations" have been developed for applicationsrequiring very high thermal insulation efficiency. These includemulti-layer metalized plastic sheets, low density foam, etc. Theseinsulation arrangements, while generally effective, tend to be complex,cumbersome and difficult to install and maintain on large structures.Foam often must be cut to shape from large blocks and installed piece bypiece. Attempts to form foam in place directly on structures has hadonly limited success due to difficulties in forming insitu foam layersof uniform low density and thickness. Spraying a self-rising foamdirectly on a structure tends to produce layers of very uneven thicknesswith an undesirable rind or skin on the outside surface which aredifficult and expensive to machine to a uniform thickness. Applying foamwithin a closed mold in contact with the structure tends to producelayers of higher density than desired, and of uneven density, since thefoam cannot fully expand. Cutting holes in the mold to permit excessfoam to extrude therefrom has not been successful since the resultinglayer has uneven density, lower near the holes and higher elsewhere.Also, the backpressure resulting from the force needed to push excessfoam through the holes tends to cause the layer to have a higher thandesirable average density. The surface of the foam in contact with themold walls tends to form an undesirable rind. Because of thesedifficulties in producing uniform foam insulation layers, many cryogenicapplications must use the much more costly and complex multi-layerinsulation systems.

Thus, there is a continuing need for improved high efficiency insulationcoatings especially for the insulation of very large structurescontaining materials at cryogenic temperatures.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide an apparatusfor forming an insulation layer overcoming the above-noted problems.

A further object of this invention is to provide an apparatus forproducing a coating of foam having uniform low density and heightcharacteristics.

Yet another object of this invention is to provide an apparatus forrapidly forming a uniform foam layer over large surfaces.

The above objects, and others, are accomplished in accordance with thisinvention by an apparatus which comprises a plurality of upstanding pinsor studs spaced across the surface to be coated, a taut screen sheetsupported by the studs at a selected height above the surface, means toapply a self-rising foamable liquid uniformly to the surface through thescreen and means to remove the screen after the liquid has foamed upthrough the screen and cured to at least a self-sustaining state.

Any suitable liquid foamable material may be used. Typical foamablematerials include synthetic resin materials which include a chemical"blowing agent" which reacts, releasing a gas which forms foam poreswithin the liquid. Other foamable liquids, for example, may includedispersed volatile liquids which form local pores upon heating orlowering of ambient pressure. The foam material itself may be anysuitable material which can be foamed in a liquid state, then cured orsolidified to a self-sustaining state. The resin may be cured, forexample, by a catalytic reaction, heat, etc. If desired, variousmaterials may be added to the foamable liquid to modify its properties.For example, finely chopped glass fibers may be added to increase thestrength of the foam.

Typical foam materials include urethane, polyethylene, vinyls, epoxys,silicones, phenolics, urea-formaldehydes, fluorocarbons, and mixturesand copolymers thereof.

The most effective foam for insulation purposes has the lowestreasonably achievable density; that is, has the greatest proportion ofsmall bubbles and smallest proportion of resin per unit volume.

"Low density" as used in this application is a relative term whichindicates a density approaching that achieved when a foamable materialis allowed to foam and increase in volume without mechanicalrestriction. Actual density achieved, of course, varies with foammaterials, pressure and temperature. Density increases when free foamingis restricted, e.g. by foaming in a closed mold or one with restrictedopenings for release of excess foam. Similarly, non-uniform restrictionson the foaming material will cause uneven final density. For example, amold with few, widely spaced, openings for release of excess foam willhave higher density away from the openings and lower density near theopenings. In an insulation system, the lowest foam density consistentwith reasonable mechanical strength in the coating is preferable formaximum insulation effect. Uneven density is also undesirable ininsulation because of the varying temperature gradients through regionsof higher and lower density and possible resulting "hot spots" over highdensity areas.

If desired, the foam layer include reinforcements or other additives,such as one or more sheets of open, net-like, scrim cloth embedded inthe foam parallel to the supporting surface, chopped fibers mixed withthe foamable liquid, plasticizers, surfactants, etc.

The foam layer may be treated in any suitable manner after the screen isstripped away. For example, the surface may be lightly sanded to removeany roughness produced by the screen, coatings may be applied to sealthe foam, cover sheets may be laminated onto the exposed surface,additional layers of foam may be applied, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate a preferred embodiment of the apparatus of thisinvention. In the drawings:

FIG. 1 is a schematic section through the screen and surface assemblytaken perpendicular to the surface prior to application of a foam layer;

FIG. 2 is a schematic illustration of the assembly of FIG. 1 after theapplication of the foam; and

FIG. 3 is a schematic illustration of the assembly of FIG. 2 showing theremoval of the screen.

DETAILED DESCRIPTION OF THE INVENTION

The three major steps in the use of the apparatus for producing uniformthickness, low density foam layers are illustrated in FIGS. 1, 2 and 3.

As seen in FIG. 1, a plurality of upstanding pins or studs 10 aresecured to the surface of structure 12 which is to be coated with a foamlayer. Studs 10 should have a height slightly greater than the thicknessof the desired foam layer, and should be spaced closely enough tomaintain a substantially even, taut screen therebetween. Typically,studs 10 may be spaced 2-4 feet apart. Any suitable material may be usedfor studs 10 and structure 12. Typical materials include metals such asaluminum or steel, synthetic resins, glass, etc. The studs may besecured to the structure surface in any suitable manner, such as bywelding, adhesive bonding, or insertion into shallow holes.

If desired as a reinforcement for the foam layer, one or more sheets ofscrim 14 may be stretched taut and slipped down over studs 10. Anysuitable material may be used for scrim, in any suitable weave. Typicalmaterials include polyester fibers, such as those available under thetrademark "Dacron" from E. I. duPont de Nemours, Inc., glass fibers,fine metal wires, nylon, etc. The fibers may be coated to increaseadhesion to the foam material, if desired. Typical mesh sizes range fromabout 10 by 10 to about 4 by 4 thread strands per inch. Preferably,thread thickness should be no greater than 0.005 inch. Up to five spacedscrim sheets, each having a maximum fiber diameter of 0.005 inch andminimum, net-like, thread spacing of no less than about 0.1 inch may beused without excessive back pressure on the expanding foam.

After the scrim 14, if any, and any other objects to be imbedded in thefoam are positioned, a sheet of screen material 16 is stretched taut andlowered over studs 10. Preferably, friction holding disks 18, which maytypically be either "Tinerman" washers, split rings or thin plasticdisks having slits or holes slightly smaller than the stud diameter, areslipped down over studs 10 to hold screen 16 at the selected distanceabove surface 12. Disks 18 have a sufficiently tight friction fit onstuds 10 to prevent screen 16 from being moved up as the foam expands.Since the upward force on screen 16 from the expanding foam is notgreat, disks 18 do not require an extremely tight fit on studs 10.

Screen 16 may be constructed from any suitable material woven in asuitable mesh size. The screen fibers should occupy less than 30% of thescreen area to provide the least possible resistance to foam expansion.Where the screen occupies more than 30% of the screen area, the foam hasbeen found to expand less than the desired amount and foam density ishigher than desired due, apparently, to resistance to flow through thescreen and resulting "back pressure" on the foam. While optimum resultsare probably obtained where the proportion of screen fibers to open areaapproaches zero, screen strength with extremely thin strands is toolittle to permit removal of excess foam by stripping away the screen.Also, if the mesh spacing is too great, an undesirably rough foamsurface will be left. The screen should have the smallest fiber diameterand mesh size consistent with providing at least 70% open area andsufficient strength for the stripping operation. Small mesh size tendsto produce a smoother final foam surface. Typical mesh sizes range fromabout 4 to about 8 threads per inch, with from about 4 to 5 threads perinch giving the optimum combination of stripping effectiveness and lowresistance to foam expansion. Fiber diameters in the 0.02 to 0.06 inchrange are typical, with the finer fibers being most effective. Typicalscreen fiber materials include metals, such as aluminum and stainlesssteel, copper, brass, monel and galvanized steel, glass, syntheticresins, such as nylon, fluorinated ethylene or propylene, natural fiberssuch as cotton and wool, and combinations thereof. Of these, galvanizedsteel wire is preferred because of its high strength, stiffness and lowcost. Rather than woven mesh screens, perforated metal sheets having atleast 70% open area and very small webs between perforations may beused, as may netting of the sort often molded or extruded from syntheticresins, such as polyolefins. If desired, the screen may be coated ortreated to reduce wetting by and adhesion to the foam material. Bestresults are obtained in general with a woven mesh of about 0.02 inchgalvanized wire mesh with a mesh spacing of about 0.25 inch, whichprovides about 80-90% open area. This provides the optimum combinationof low resistance to foam flow, sufficient screen strength for thestripping step, and sufficiently small mesh size for a smooth foamsurface.

While the foamable liquid may be applied in any suitable manner,spraying of foamable material 20 by spray head 22 through screen 16 ispreferred. As sprayed, foam material 20 is in a very liquid, lowviscosity state, which passes through screen 16 and scrim 14 withoutbuilding up thereon. The foam then expands, due to a chemical reactionor other gas bubble producing mechanism. Spray head 22 is moved relativeto surface 12 as indicated by the arrow adjacent to head 22 in FIG. 1,to deposit a substantially uniform layer of foamable liquid over surface12. Spray head 22 and/or surface 12 may be moved in any suitable patternto provide this approximately uniform application. As seen in FIG. 1,with rapidly expanding material, the foam forms immediately behind sprayhead 22. More slowly foaming materials would form a liquid foamablematerial layer over surface 22 and would foam more levelly, so that thefoam slope shown in FIG. 1 would be more gradual.

As seen in FIG. 2, foam 20 expands up through scrim 14 and screen 16,finally forming an irregular upper surface with a thin skin or rind ofcured material on the outer surface. Scrim 14 may be carried upwardlyvery slightly by the rising foam. If the final position of scrim isimportant, experience will teach the level at which scrim 14 should bepositioned to give the desired final position for a particular foamcomposition.

Once foam 20 has cured to a self-sustaining, shape-retaining state,screen 16 can be removed. Of course, foam 20 can be completely curedbefore screen 16 is removed. If "Tinerman" washers are used forretaining disks 18, conventional end cutters are used to clip off theends of studs 10 adjacent to disks 18. The cutters can simply be pusheddown through foam 20 along studs 10 until disks 18 are touched, then thestuds are snipped off. If disks 18 are, for example, slit plastic disksor split rings which are reasonably freely slidable along studs 10, theends of the studs need not be removed.

The excess material extending through the screen may be removed in anysuitable manner. With many foam materials, the screen is preferablystripped away after the foam has reached a shape-retaining,self-sustaining state but before the foam is fully cured, to preventspalling or tearing of the foam layer which may adhere too strongly tothe screen when fully cured. The screen may be removed by lifting oneedge, then rolling the screen over the surface to pull the screen awayprimarily in tension. Alternatively, the screen may first be movedslightly in a direction parallel to the desired foam surface to shearthe foam portions extending through the screen. For example, with 0.25inch spaced grid (mesh), the screen might be moved about 0.3 inchparallel to the surface to shear foam extruding through the mesh, thenbe lifted away. This technique often produces a smoother foam surface.If desired, the foam extending through the screen may at least partiallybe removed prior to removal of the screen. Typically, after partial cureof the foam, a doctor blade or rotating stiff brush may be moved alongthe upper screen surface to remove at least part of the excess foam.

As seen in FIG. 3, screen 16 and the portion of foam 20 above the screenare removed in the embodiment shown by manually stripping the screenaway. Typically, a bar 23 having a length substantially equal to thewidth of screen 16 and having a plurality of hooks 25 in line along thebar may be used for removing the screen. Either manually or mechanicallyhooks 25 are pushed through the excess foam and into engagement withscreen 16 near one end thereof. Bar 23 is lifted and pulled back to liftand roll back screen 16 and excess foam thereon. Alternatively, bar 23could be rotated to roll up screen 16 and excess foam.

Pre-clipped studs 10 will protrude above foam 20 only very slightly.Unclipped studs 10, or the slight stub remaining after pre-clipping canbe removed, if desired, with conventional wire cutters. If desired,disks 18 may be replaced on unclipped studs 10 after screen removal toact as secondary foam retainers in case of bond failure between the foamand surface 12. Most foam materials will automatically bond securely tosurface 12 as the foam cures.

The substantially smooth foam surface can be treated in any suitablemanner. After screen stripping, the foam surface will be slightly rough,with slight peaks or depressions within the area of each screen opening.This roughness may be desirable as an excellent bonding surface againstwhich sheet materials can be adhesively laminated. If desired, thesurface can be lightly sanded for improved smoothness.

OPERATION

The operation of a preferred embodiment of the apparatus of thisinvention will be illustrated by the following example. Parts andpercentages are by weight unless otherwise indicated.

A number of 2-inch long, 0.07 inch diameter, aluminum pins or studs arewelded to a large aluminum plate at the corners of a 3-foot square grid.A sheet of nylon scrim having an average strand diameter of about 0.005inch and grid size of about 0.25 inch is stretched taut and slipped overthe studs to about 1/2 inch above the plate. An aluminum screen havingstrand diameters of about 0.028 inch and a grid size of about 0.25 inchis stretched taut and slipped over the studs. A Tinerman washer isslipped over each stud and pressed down until each is about 1 inch abovethe plate. A foamable liquid, Stepin BX-250A, is prepared to the properspray consistency. A substantially uniform layer of the foamable liquidis sprayed through the screen onto the plate with a Gusmer spray gun,proportioning unit Model FF, manufactured by Gusmer, Inc., with 52.17 to47.83 ratio pumps. The spray gun is moved in a scanning pattern over thescreen to provide uniform application of the liquid to the plate surfacethrough the screen and scrim. The self-rising foam is allowed to riseand cure to the point where it is self-sustaining. An irregular foamsurface extends above the screen. End-cutting wire cutters are slippeddown through the foam over each stud and each stud is clipped offadjacent to the Tinerman washers. The screen is lifted and rolled back,causing the washers to pop off. The resulting foam layer has uniformthickness and uniform low density. The surface is level but moderatelyrough. Light hand sanding with fine sandpaper fastened to a blockproduces a smooth, even surface.

While certain specific materials, components and arrangements weredescribed in detail in the above description of preferred embodiments,these may be varied and other components and ingredients may be used,where suitable, with similar results. Various modifications,applications and ramifications of the present invention will occur tothose skilled in the art upon reading the present disclosure. These areintended to be included within the scope of this invention, as definedin the appended claims.

I claim:
 1. Apparatus for forming a uniform layer of foam on a surface which comprises:means securing a plurality of spaced upstanding thin studs to a surface to be covered with a foam layer; a flexible screen having a substantially uniform pattern of openings, said openings comprising at least about 70% of the screen area; said screen being taut and spaced a selected distance from said surface, with said studs extending through said screen and frictionally supporting it in place; friction disks over at least some of said studs in contact with said screen to prevent movement of said screen along said stud away from said surface; means for applying foam material in a liquid, unexpanded state through said screen to form a uniform layer over said surface; and means for removing said friction disks, and said screen after the foam has risen up through said screen and at least partially cured to a shape-retaining state.
 2. The apparatus according to claim 1 wherein said screen comprises fibers having diameters of from about 0.02 to 0.06 inch and mesh sizes in the range of about 4 to 8 fibers per inch.
 3. The apparatus according to claim 1 wherein said means for applying foam material comprises at least one spray gun adapted to be moved in a uniform pattern across said surface to apply a uniform coating of foamable liquid by projecting said liquid against said surface through said screen.
 4. The apparatus according to claim 1 wherein said screen comprises a galvanized steel wire mesh having wire diameter of about 0.02 inch and a mesh wire spacing of about 0.25 inch. 